An offer of free anti-virus software is essentially a marketing technique...a way of advertising and enticement to get folks to try a product and if they like it, to purchase the full (or Pro) version which typically provides more features. Marketing and promotional strategies are built into the vendor's business model as part of their operating costs. Bottom line...it's all about generating revenue and finding new and creative ways to do so. As such, users may have to deal with occasional nagging pop-ups or nuisance advertising and prompts to upgrade to the paid version.

By using such free programs, you are essentially agreeing to the terms of the vendor's service which includes those ads. In some cases you may be able to disable annoying pop-ups through the anti-virus settings if the vendor included an option to disable them. If the vendor does not have such an option listed, then your only alternative is to switch anti-virus software if the pop-up ads annoy you that much.

Also keep in mind that many anti-virus vendors are bundling toolbars and other software with their products as a cost recoup measure. In fact, all free Anti-virus programs now come with toolbars or other bundled software except Bitdefender Free, Microsoft Security Essentials and Windows 8/10 Defender.

I have never used Qihoo 360 Total Security before but I have used Avast Free. I did not encounter an issue with scheduled scans on my Win 7 Ultimate x64 partition with Avast Free. The combination there is:

Among the free AV I used with that same combination was Avira Free and AVG Free. Last was AVG Free. I uninstalled AVG Free and installed Avast Free because since Avast has bought AVG it's not feasible anymore to run AVG as the engine will soon e incorporated.

Yeah. Avast Free is very reliable and have been stable since I first used it. Does what it supposed to. All of my free setups have used Avast Free. As to Qihoo well have not really been convinced with it although the reviews are good and some of the malware testers use it too. To me still not gonna dive in with that crowd.